Not mentioning the ferret in your rental agreement unfortunately doesn't help you much. Landlords can generally restrict or prohibit pets, and that applies even more to animals that aren't explicitly allowed. With dogs and cats, there are now more legal protections in place, but ferrets often fall into a gray area - some courts view them as potential pests (which is biologically questionable, but legally it can definitely be relevant).
Still, it's worth trying to talk to your landlord. Often it helps to simply show concretely that your ferret is neither particularly dangerous nor particularly loud and that you have everything under control. Some landlords are more flexible when you demonstrate that it's not a major problem. You could also ask whether he'd accept a deposit for potential damages or want to put an agreement down in writing.
Legally speaking though, it's tough for you. If things escalate, you can contact a tenant association - they know the current case law in your state better. But honestly: it'll probably be worth more to sort things out before it turns into a real conflict.